Raster scan caption generators are useful in television receivers for displaying various types of information such as teletext data, time and channel settings, computer data and so on. In a typical generator individual alphanumeric characters or graphic symbols are represented by a dot matrix pattern stored in a read only memory (ROM). A character is generated for display by transferring a desired dot pattern from the ROM to a high speed buffer and sequentially shifting the character "dots" or "pixels" (picture elements) out of the buffer with a pixel or dot clock. The serial signal, thus formed, is applied to a kinescope in a timed relation to the vertical and horizontal sweep so as to display the dot matrix pattern at a desired location on the raster.
It is known that the clock which determines the pixel timing may be either "line-locked" or "burst-locked". A line-locked clock is one which is synchronized to operate at a frequency which is an integral multiple of the horizontal line frequency. The advantage of line-locking the pixel clock is that each character dot of the caption signal will have a stable position on a displayed line even though the line frequency may vary. If, for example, the line frequency decreases making the sweep longer, the dot clock frequency will also decrease proportionately, thereby increasing the pixel period to compensate for the longer sweep time. The net effect for a line-locked pixel clock is that variations of the pixel width and position due to variations in the horizontal line frequency are relatively unnoticeable. This is a distinct advantage when displaying captions along with a video signal provided by a source which may have a non-standard or unstable line frequency (e.g., a video tape recorder, video disc player, etc.).
Notwithstanding the advantages of line-locking the character generator dot clock, there are applications where it is advantageous to employ "burst-locking" instead. In a burst-locked system, the clock used for pixel timing is locked to a multiple of the color subcarrier frequency by means of a burst keyed phase locked loop (PLL). An advantage of burst locking is that conventional color television receivers include a color oscillator locked to incoming burst thereby eliminating the need for a separate high frequency dot clock for the character generator. The color subcarrier reference frequency is also useful for decoding teletext data transmitted during the vertical blanking interval of received video signals. Burst-locked clocking is particularly important in digital television receivers where the composite video signal is sampled and converted to digital form for processing. In such systems the use of a clock locked to the color subcarrier reference frequency aids in minimizing cross-color effects and other undesirable artifacts in displayed images and simplifies chroma demodulation. An integrated circuit which includes a teletext decoder and a character generator that is switchable between teletext input provided by the teletext decoder and an external data input and which is intended for use with a burst-locked clock is described at pages 139-141 of the data book entitled "DIGIT 2000 VLSI DIGITAL TV SYSTEM" published September 1981, by Deutsche ITT Industries GmbH.